All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If the situation is as you mentioned in the question, that prohibitions are taking place in this school and that you fear about your own self or your religion, then it is not permissible for you to obey your parents in going to that school, because obeying parents is only in what is permissible. The Prophet said: “There is no obedience to a creature in disobeying Allaah, obedience is required only in what is permissible.” [Al-Bukhari].
There is no doubt that it is a disobedience to go to a school where acts of disobedience take place, like mixing between boys and girls, girls not wearing Hijab, and the like; a Muslim is required to stay away from sins and the sinners and the place where the sin takes place.
Finally, we advise you to be kind to your parents and advise them and try to convince them that it is forbidden to go to that school and that you fear for your religion. You should do so in a wise and soft manner which befits the status of parents.
For more benefit, please refer to Fatwa 84942.
Allaah Knows best.